There is considerable variation in primate species richness across neotropical forest sites, and the richest assemblages are found in western Amazonia. Forest type is an important determinant of the patterns of platyrrhine primate diversity, abundance, and biomass. Here we present data on the assemblage structure of primates in adjacent unflooded (terra firme) and seasonally inundated (va ´rzea and igapo ´) forests in the lower Puru ´s region of central-western Brazilian Amazonia. A line-transect census of 2,026 km in terra firme, 2,309 km in va ´rzea, and 277 km in igapo ´was conducted. Twelve primate species were recorded from 2,059 primate group sightings. Although terra firme was found to be consistently more species-rich than va ´rzea, the aggregate primate density in terra firme forest was considerably lower than that in the species-poor va ´rzea. Consequently, the total biomass estimate was much higher in va ´rzea compared to either terra firme or igapo ´forest. Brown capuchin monkeys (Cebus apella) were the most abundant species in terra firme, but were outnumbered by squirrel monkeys (Saimiri cf. ustus) in the va ´rzea. The results suggest that floodplain forest is a crucial complement to terra firme in terms of primate conservation in Amazonian forests.